Life gets pretty boring when you can beat the snot out of any villain with just one punch. Nothing about Saitama passes the eyeball test when it comes to superheroes, from his lifeless expression to his bald head to his unimpressive physique. However, this average-looking guy has a not-so-average problem-he just can't seem to find an opponent strong enough to take on! S-class hero Blast suddenly appears before Saitama and Flash in search of a mysterious box! But what is it about the box that has the Hero Association's top hero chasing after it? Meanwhile, several heroes are locked in a fierce battle with Monster Association executives when Genos's power core becomes unstable!